On a packing machine, each article is fed by a succession of packing conveyors along a packing path to be paired with a respective sheet of packing material, which is subsequently folded about the article to define a package. The package must be stabilized by permanently joining, by gluing or heat sealing, overlapping portions of the sheet of packing material. More specifically, when gluing is employed, a layer of glue is deposited on a number of portions of the sheet of packing material before the sheet of packing material is folded about the article.
Hot-melt glue, i.e. glue which is solid at ambient temperature and becomes fluid when heated, is known to be applied to the sheet of packing material long before the sheet of packing material is folded about the article, so as to allow the spots of glue on the flat sheet of packing material to set; and, once the package is completed by folding the sheet of packing material about the article, the spots of glue deposited beforehand are reactivated, i.e. melted, by heating the portions of the package at the glue spots. The above method of reactivating the glue spots after the package is completed has the advantage of enabling the gumming devices to be set up in the best location (i.e. for easy access and cleaning) and not necessarily immediately upstream from the folding members where space is normally limited. Moreover, in the event of temporary stoppage of the machine, the sheets of packing material on which glue has already been deposited, and which have not yet been folded, do not result in faulty packages when the machine is started up again.
On the other hand, reactivating the glue spots after the package is completed complicates the structure of the packing conveyors, in that the seats on the packing conveyors carrying the packages must be provided with heating members to heat the packages at the glue spots. Normally, a packing conveyor seat is equipped with electric resistors, which are powered by a fixed external electric power supply by means of brushes integral with the packing conveyor and which slide over a fixed contact positioned parallel to the path of the packing conveyor. This solution has the drawback of requiring periodic replacement of the brushes, which wear out relatively fast.
It has also been proposed to heat the heating members by magnetic induction. Such a method, however, cannot be used for packages comprising sheets of metalized packing material, in that the magnetic flux for heating the heating members tends to also heat and irreparably damage the sheets of metalized packing material.